PWM is an effective technique to control an analog circuit using digital outputs of a microprocessor, and it is widely used in many fields, such as fields of measurement, communication, power control and conversion.
Referring to FIG. 1, in the prior art, a PWM circuit 10 generally includes: a PWM counter 11 adapted to perform up/down counting for a clock signal; a reference value setting register 12 adapted to set a comparison reference value which determines the duty cycle of a PWM signal; and a comparator 13 adapted to compare the count value of the PWM counter with the comparison reference value set by the reference value setting register and generate an effective PWM signal.
In LED display screen, the PWM technique is adopted in order to improve the display resolution. Usually the PWM is 16 bit. In the conventional LED display screen, a serial bus including SDI (Serial Digital Input), DCLK (Serial Data Input Clock), LE (Latch Enable), GCLK (Graph Clock Input) and SDO (Serial Data Output) is usually adopted, in which the GCLK is an input clock of the PWM and the frequency of the GCLK is less than 30 MHz. There are two types of the LED display screen: dynamic display screen and static display screen. For the static display screen, each LED pixel is driven by a separate constant current source; whereas for the dynamic display, time-sharing driving is adopted, in which LEDs of each column share the same constant current source for driving. The display brightness of the LED is determined by the duty cycle output by the PWM.
In a dynamic scanning system, a display refresh frequency greater than 2 KHz is required. And generally, the PWM has a resolution of 16 bit. Due to effect of the frequency of the GCLK, the number of GCLKs for forming a pulse cycle of a 16 bit PWM is: 216=65536. If the frequency of GCLK is 30 MHz, then the length of one cycle of the 16 bit PWM is: 65536/30M=2.19 ms; and the refresh frequency is 458 Hz. If the dynamic scanning is “4-scan”, then the refresh frequency of the LED display screen is 458/4=114.5 Hz. In order to meet practical requirements of application and increase the dynamic refresh frequency, it can only reduce the cycle length of the PWM, i.e., reduce the bit length of PWM. For example, if the bit length of the PWM is 12 bit, the refresh frequency may be 1824 Hz. However, the reduction of the bit length may cause a decrease in resolution, therefore the contradiction between the refresh frequency and the resolution of the dynamic display screen can not be solved in the conventional solution.